EFFECTS OF MORPHINE ON DIFFERENT ASPECTS OF SOCIAL PLAY IN JUVENILE RATS

To clarify the influence of opioids on social play, the effects of mor phine on playful and non-playful social behavior in juvenile rats was investigated under different conditions. Environmental variables emplo yed were different (dim and intense) levels of illumination during tes ting, familiarity to the test cage, and different periods of social is olation prior to testing. Under dim light conditions, morphine markedl y increased playful social behavior, such as pinning, boxing/wrestling and following/chasing, whereas non-playful social behavior such as so cial exploration and contact behavior was hardly affected. This effect of morphine was independent of duration of previous isolation and dos e-dependent, with a maximal effect at 1.0 mg/kg. The mechanism of this effect is interpreted as an action on the rewarding aspects of play. A dose of 0.1 mg/kg of morphine abolished the initial suppression of p lay induced by unfamiliarity to the test cage, without influencing tot al levels of play. This may be an effect of morphine on the integratio n of sensory stimuli. Under intense light conditions, where playful be havior was completely suppressed, morphine itself hardly affected such behavior, but decreased some aspects of non-playful social behavior. These results suggest that in juvenile rats playful and non-playful fo rms of social behavior are differentially regulated. In addition, opio id systems may be involved at different levels in the regulation of so cial play.